Sun lamp and Diffuse surface is not lambertian?
Assuming the sun light has size 0 and strength 1, the correct maximal value on the sphere is 1&frasl;pi, because of energy conservation and the units used.

Cycles uses physically based units for light strength, in this case Watts&frasl;m2 for sun lights. Other renderers and game engines (typically ones that were not designed with physically based rendering in mind) often use a light strength that is implicitly multiplied by pi, which would give the maximal value of 1 you expect.